Synapse

2 Pages 483 Words

A chemical synapse consists of a neuronal presynaptic terminal, a synaptic cleft and a postsynaptic membrane with associated receptor proteins. The chemical synapse is highly developed in the CNS. It conducts the signal only one way, and has a synaptic delay.
The presynaptic axon terminal typically broadens to form a bouton terminaux (presynaptic terminal). The action potential, which originates in the CNS, depolarises the axon membrane by an influx of Na+. Repolarization follows rapidly by selective K+-efflux When the action potential reaches the presynaptic membrane, Ca2+ enters the terminal through voltage-gated Ca2+-channels. Vesicles containing transmitter, fuse with the presynaptic membrane and release their contents of acetylcholine into the synaptic cleft causing exocytosis. Acetylcholine, diffuses across the synaptic cleft and binds to specific receptors, which are located into the postsynaptic membrane. This ligand binding elicits a transient opening of pores, which are specifically permeable to small cations. The synaptic cleft of a chemical synapse is about 30 nm. The ACh-receptor opens and allows influx of Na+, whereby the membrane depolarizes and an action potential is generated which propagates along the length of the postganglionic axon. This is an appropriate response of the postsynaptic cell to the received signal. The effect is terminated by the enzyme acetylcholinesterase, which hydrolyses acetylcholine into two inactive products acetic acid and choline. Influx of Na+ or efflux of K+ through the pores of the receptors changes the postsynaptic membrane potential. If the presynaptic action potential (AP) results in a postsynaptic depolarization, it is called an Excitatory Post-Synaptic Potential (EPSP). If the AP results in a postsynaptic hyperpolarization, then it is called an Inhibitory Post-Synaptic Potential (IPSP). Excitatory synapses often use glutamate as the transmitter. The pores are penetrated by Na+, w...